1. Field of the Invention
The present invention relates generally to computer chips, and more particularly to ball grid arrays for attaching computer chips.
2. Description of the Related Art
Conventionally, electronic circuit components, including silicon chips, have been mounted on printed circuit boards. More recently, in order to reduce the size associated with conventional printed circuit boards, low-temperature co-fired ceramic (LTCC) substrates have been used.
A typical LTCC configuration comprises multiple layers of ceramic xe2x80x9ctapexe2x80x9d which are used to provide the base structure upon which to form various electronic components and electrical connections. The tape is formed from a powdered ceramic, mixed with a binder. For example, one type of ceramic tape available from Dupont is known as xe2x80x9cGreen Tape 951.xe2x80x9d The electronic components that can be formed include resistors, capacitors, inductors, and the like. The electrical connections, formed through each tape layer are known as xe2x80x9cvias.xe2x80x9d The components are formed by punching holes in the tape as appropriate, and layering on metal, dielectrics, insulators, etc. Several layers of tape may be used in order to form the desired circuitry. The tape layers are then pressed together and fired in an oven to remove the binder and to sinter the ceramic powder. Components which are too large or too difficult to form within the ceramic tape layers, such as silicon chips, may be surface mounted on the hardened substrate. The resulting substrate is usually less than 1xe2x80x3xc3x971xe2x80x3 thus providing a compact circuit package.
U.S. Pat. No. 5,442,852, entitled xe2x80x9cMethod of Fabricating Solder Ball Arrayxe2x80x9d, by Paul Danner, discloses a method of forming a ball grid array on a ceramic substrate. To facilitate discussion, FIG. 1 shows an example of solder ball 100 being mounted on a ceramic substrate 104. A via 108 filled with an electrically conductive material may pass through the ceramic substrate 104. A termination pad 112 of an electrically conductive material may be placed over the via 108 to provide an electrical connection with the via 108. A dielectric tape 116 having holes matching the placement of the termination pads 116 on the ceramic substrate 104 may be mounted on the surface of the ceramic substrate 104 with the termination pad 116. The ceramic substrate 104 and dielectric tape 116 may be fired together. The solder ball 100 may be placed in the hole(s) in the dielectric tape 116. After the solder balls 100 are placed, the solder balls 100 may be reheated to reflow the solder balls 100 so that the solder balls 100 fill the volume created by the hole in the dielectric tape 116 and bonds with the termination pad 112, as shown in FIG. 2. The solder balls may be used as a ball grid array. Such ball grid arrays are subject to thermal stress and other environmental factors, which may cause a mechanical or electrical connection failure where the solder ball is connected to the termination pad.
It would thus be desirable to provide an enhanced ball grid array attachment between the solder balls and the termination pad.
In general, the present invention provides a method of forming a substrate with a ball grid array. Generally, a plurality of termination cups is formed on the substrate. A plurality of diffusion barriers is formed where each diffusion barrier is over a termination cup. Electrically conductive balls are then connected to the diffusion barriers.
These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.